Loud + ClearWhat you may not know about emergency communications systems

NFPA Journal®,May/June 2012

The 2010 edition of NFPA 72®, National Fire Alarm and Signaling Code, contains a new Chapter 24 that presents the requirements for emergency communications systems. The chapter contains requirements for all voice communications systems, including in-building fire emergency voice/alarm communications systems (EVACS) and in-building mass notification systems (MNS). The requirements range from the subtle to the not-so-subtle and will probably challenge what you thought you knew about voice communications systems design. You will need to learn these requirements in order to have success as a designer, installer, and inspector of emergency voice/alarm communications systems.

Of the various requirements, ensuring intelligibility presents the biggest challenge for both new installations and retrofits of in-building EVACS and MNS. The code defines intelligibility as “capable of being understood; comprehensible; clear.” Many misconceptions exist about this subject, especially regarding the equipment necessary to meet these requirements. Paragraph 24.4.1.2.2.1 requires that “the speaker layout of the system shall be designed to ensure intelligibility and audibility,” that “intelligibility shall first be determined by ensuring that all areas in the building have the required level of audibility,” and that “the design shall incorporate speaker placement to provide intelligibility.”

Because most fire alarm system designers, authorities having jurisdiction, and installers are unfamiliar with basic communications concepts, they don’t really understand what hints the above requirements provide. One very important hint: Intelligibility begins with speaker placement and choice. In fact, Annex A.24.4.1.2.2.2 of the code provides clear guidance for speaker placement: “Generally speaking, in a standard building configuration with normal ceiling height, normal ceiling construction (such as drop acoustical ceiling tiles), standard wall configurations, and finishes and carpeted floors, ceiling-mounted speakers should be installed in all normally occupiable spaces and in corridors spaced at a maximum of twice the ceiling height or as determined by a commercially available computer acoustical/speaker modeling program.…” In a corridor with a standard ceiling height of 10 feet (3 meters), for example, an installer would have to place the speakers 20 feet (6 meters) apart. This is a much closer spacing than that recommended for smoke detectors in a similar corridor — typically 42 feet (13 meters) apart. That means any given corridor could have significantly more speakers for a new in-building fire emergency/voice communications system than smoke detectors.

When you retrofit a standard, non-voice fire alarm system with a new in-building fire emergency/voice communications system, you cannot simply replace the horns of the old system with speakers on a one-to-one basis. You won’t have enough speakers to get you even close to the code-required intelligibility. Similarly, you cannot simply increase the power output of improperly spaced speakers to achieve intelligibility by changing the tap on the individual speaker transformers. Intelligibility normally requires a larger number of lower-output speakers properly spaced throughout the protected space. As Annex A explains, “Connecting to a high setting to meet the audibility requirements of the code could distort” the signal’s intelligibility.

These issues related to intelligibility should convince all code users that they are responsible for pursuing the training and education they will need to successfully design, install, and inspect these important systems.